In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
Optical sensors are important, light sensitive devices that are used in light detection. They can, however, be damaged if they are exposed to a high enough intensity of light. Optical limiters allow transmission of light at a low intensity and allow limited transmission of light at higher intensities. Maintaining transmitted light below a certain threshold value contributes to protection of light sensitive elements such as optical sensors and the human eye.
Active and passive forms of optical sensor protection are known. One form of sensor protection may include, for example, special glasses that a welder uses to protect the welder's eyes from the high intensity light generated by the welder's arc. Active optical limiters may include an optical sensor that can sense high intensity light and communicate electronically to an actuator to block at least some of the light, thereby reducing its intensity and minimizing damage to the sensor. Passive optical limiters are based on a wide variety of nonlinear optical phenomena, such as reverse saturable absorption, two-photon absorption, thermal lensing effects, optically-induced molecular reorientation in liquid crystals.
Optical limiting devices placed between the light source and the sensor can provide the sensor with protection from high intensity light. These protective devices absorb or scatter high intensity light while transmitting low intensity light from the light source to the sensor. No communication from the sensor or from any other device occurs in a passive optical limiting device for the device to regulate transmitted light. The increased speed is particularly important for applications where the sensor may be exposed to sudden bursts of high intensity light. Optical limiting devices are used, for example, to protect optical sensors in pulsed laser systems.
There are a variety of other materials that have been used as optical limiters, including various chemical compounds and even empty-cage fullerenes, e.g., classic fullerene structures where the interior space is empty. Some devices employ an active system that senses light intensity and controls a variable attenuator to limit the transmission through the device. A review of optical limiting devices may be found in Lee W. Tuft, et. al., “A Review of Optical Limiting Mechanisms and Devices Using Organics, Fullerenes, Semiconductors, and Other Materials,” Prog. Quant. Electr, 1993, vol. 17, pp. 299-338, the contents of which are herein incorporated by reference.
There are four main performance criteria for which prior materials for optical limiters fall short. First, these materials often do not have sufficiently high nonlinearity to limit the transmitted light intensity to below the levels at which the protected optical equipment would be damaged. Second, material response is often slow, making such materials ineffective in blocking very short high-intensity pulses. Third, such materials should, but often do not, recover very rapidly after a limiting event in order to minimize the impact of the event on normal operation of the system. Fourth, the materials ideally would transmit nearly all of the light at low intensity, and therefore would not degrade the normal operation of the system. In practice, these four performance criteria are often not met in a single optical limiter device.